Manufacture of a filtration membrane

ABSTRACT

A filtration membrane made by a method that includes: a) selecting and preparing an organic polymer, such as a collodion; b) injecting a collodion into at least one channel of an extrusion die that also comprises an extrusion die core and at least one outlet; c) injecting an internal liquid into a hollow centering pin, the hollow centering pin comprising a channel positioned on the core of the extrusion die and also positioned at an axis of the outlet of the extrusion die; d) applying a holding film to the outlet of the extrusion die; e) unrolling the holding film onto a surface of at least one hollow fiber emerging from the outlet of the extrusion die; f) immersing the hollow fiber with the first holding film in a rinsing solution so as to obtain a flat hollow fiber filtration membrane; and ending the rinsing of the filtration membrane.

TECHNICAL FIELD

The present invention relates to the field of membrane filtration andmore particularly to the manufacture of a hollow fiber filtrationmembrane associated with a support film.

TECHNICAL BACKGROUND

An important aspect of the manufacture of this type of membrane isrelated to the quality of the inner and outer surfaces of the hollowfibers and the fragility of the resulting membranes. Therefore, a methodis usually used that includes particularly delicate steps in order toavoid mechanical stress and physical damage to the membranes.

PRIOR ART

The manufacture of organic membranes is based on the principles of phaseseparation (dry, wet, thermal) or foaming, and even irradiation followedby appropriate chemical treatment(s). The steps in manufacturing amembrane produced according to the principle of liquid-liquid phaseseparation are seemingly simple. But in reality, it is a complex processto manufacture a membrane because it is possible to obtain membranesthat perform very differently, from a same formulation (collodion).Indeed, the geometry of the spinneret and/or casting device used, thethermal conditions, the nature of the precipitation liquid, and the drawconditions are all factors that influence the ultimate performance ofthe membrane.

Conventionally, production of a membrane in the form of a hollow fiberincludes several steps such as extrusion of the fiber, rinsing,posttreatment, and winding or bundling.

In general, an extrusion spinneret places a collodion in contact with acentering or precipitation liquid. The collodion is forced to dividearound a centering needle, into which is injected either a precipitation(or coagulation) liquid in order to form a fiber having an inner skin,or a centering liquid in order to form a hollow fiber having an outerskin. The flow rate and temperature of the collodion and the internal(bore) liquid supplied to the extrusion spinneret are controlled.

According to this protocol, the nascent fiber falls into a bath whichserves to precipitate the outer surface of the fiber in the case of ahollow fiber having an outer skin, or to begin rinsing it in the case ofa hollow fiber having an inner skin. The nascent fiber thus formed isadvanced to a rinsing bay. Rinsing the fiber may be followed byposttreatment that preserves its hydraulic performance as it dries, astep necessary for fiber bonding. The fiber is then wound onto a spoolfor later use or is directly bundled into a specific form.

When producing a hollow fiber through an extrusion spinneret, thesurface state of the centering needle and, more importantly, the surfacestate of the extrusion orifice can cause the formation of major defectsin its inner and outer surface. Experience has shown that certaindefects on the external portion of the centering needle have little orno influence on the surface state of the inner skin of the fiber. Incontrast, it has been noted that an even slightly degraded surface ofthe extrusion orifice can cause major defects in the outer surface ofthe fiber.

The presence of the slightest defect on the inner perimeter of theextrusion orifice of a spinneret can therefore significantly impact thequality of the surface of the nascent fiber. In fact, the swelling whichoccurs in the nascent fiber increases the impact of existing defects onthe perimeter of the extrusion orifice. Different types of surfacedefects or problems are seen on the outer surface of a fiber:rectilinear indentations following the direction of flow, flaw marks,and tears. It is difficult to change the manufacturing conditions tominimize the impact of the surface state of the spinneret on the defectsformed on the outer surface of the fiber. To correct the problem, it isessential to constantly resurface the face of the extrusion orifice.Generally, the extrusion spinneret must be changed, which considerablyincreases the manufacturing cost of the membranes.

When it exits the spinneret, the fiber is then passed through aprecipitation or rinsing bath in which it is in contact with take-uprollers and drive rollers ensuring the continuity of the spinningoperation. The outer surface of a hollow fiber or the active surface ofa flat membrane is often damaged by the presence of scratches or burrson the face of the advancement means which come into contact with them.The surface state of these elements must therefore be checkedperiodically and the surfaces concerned must be corrected.

At this stage, the fiber, still charged with solvent, is drawn at a ratethat is greater than the natural rate of extrusion in order to managespinning continuity. Some membrane manufacturers take advantage of theinfluence of such drawing on the porous structure of the membrane, togive the pores a particular shape. The membranes so produced have asemicrystalline structure, which can accelerate their aging and therebylimit their attraction.

Depending on the surface state of the rollers and on the draw conditionsimposed, the fiber may suffer damage which can be very significant.Observations made by scanning electron microscopy have shown that theouter surface of a fiber or the active surface of a flat membrane isvery often damaged: indentations, marks, and tears are present on aregular basis. These major defects affect the retention performance ofmembranes produced in this manner. Experience has shown that thesedefects are the cause of accelerated chemical, mechanical, or thermalaging of membranes.

The production of a membrane without such defects would not only improveits initial separation performance, but would also give it betterstability with respect to chemical, thermal, and mechanical aging.

Conventionally, the nature of the external bath also has an influence inthe production of hollow fibers. As the fibers are produced at spinningspeeds ranging from several meters to several tens of meters per minute,the time the nascent fiber remains in the air is very short. Severalparameters of the conventional method for manufacturing hollow fiberscan be adapted to overcome this disadvantage.

It is possible to make use of the more or less slow precipitation of thecollodion so as to form a true filtering skin on the outer surface ofthe fiber. Some manufacturers use this property to produce “double skin”membranes. However, the presence of this dual filtration barrier resultsin poor hydraulic performance. In addition, as previously indicated, thepresence of numerous advancement means creates surface defects that candegrade the separation performance of the outer skin of the membrane.

Another possibility is to vary the concentrations of pore formingagents. The use of collodions with a high concentration of pore formingagents has been proposed to bring the support polymer to the phaseseparation limit. This then results in the formation of hollow fiberswhich have attractive water permeabilities but poor selectivityperformance. Indeed, this type of membrane has filtration pores with awide pore size distribution. Often these collodions also containmicrogels which are the cause of many defects affecting selectivity andaccelerating the chemical, thermal, and mechanical aging of themembranes so produced.

The use of collodions with a reduced concentration of pore-formingagents allows obtaining relatively stable collodions, a key factor inensuring reproducibility in the manufacture of membranes. However, thespinning speeds are limited with such collodions. To maintain productioncapacity, we must therefore increase the number of fibers produced inparallel, which intensifies spinning difficulties and gives rise toadditional problems.

In addition, flat and hollow fiber membranes produced with an identicalformulation did not provide the same hydraulic performance. Generally,flat membranes produced on a woven or nonwoven support have a waterpermeability that is improved by a significant factor (2 to 5). A flatmembrane can thus have a water permeability of 1000 l/h·m²·bar while ahollow fiber produced under the same precipitation conditions (samecollodion, same precipitation liquid, and same temperature) has a waterpermeability of close to 200-300 l/h·m²·bar.

SUMMARY OF THE INVENTION

To overcome some or all of the disadvantages of the prior art describedabove, a method is provided for the rapid manufacture of a filtrationmembrane, that is economical in materials and provides goodreproducibility. The filtration membrane resulting from this method isparticularly resistant and efficient.

More specifically, the method for manufacturing a filtration membranecomprises the following steps:

a) selecting and preparing an organic polymer such as a collodion,b) injecting a collodion into at least one channel of an extrusionspinneret that further comprises an extrusion spinneret core and atleast one outlet,c) injecting an internal liquid into a hollow centering needle whichcomprises a channel positioned at the core of the extrusion spinneretand which is positioned at an axis of the outlet of the extrusionspinneret,d) applying a support film at the outlet of the extrusion spinneret,e) unrolling the support film onto a surface of at least one nascenthollow fiber emerging from the outlet of the extrusion spinneret,f) immersing the hollow fiber with the first support film in a rinsingsolution so as to obtain a flat hollow fiber filtration membrane,g) ending the rinsing of the filtration membrane thus obtained.

Generally, in order to shape this membrane successfully, injection of acentering fluid containing excess solvent precedes injection of thecollodion. This precaution prevents the centering needle from cloggingdue to capillary creep of the collodion, which would require cleaningthe spinneret and restarting the spinning process.

There are several advantages of the method so defined, as will bedetailed below.

As explained earlier, traditionally the manufacture of a hollow fiber isgenerally accompanied by multiple areas of damage to the fiber itself,whether due to the surface state of certain parts of the extrusionspinneret, the drive rollers, the drawing of the fibers, or the natureof the external bath. The method of the present invention overcomesthese disadvantages by associating a support with the hollow fiber,serving to reinforce and protect the hollow fiber during themanufacturing process without impacting its filtration capabilities.

Supported flat membranes have already been described in the prior art.In those examples, drawing of the supported membrane is managed bypulling the woven or nonwoven support onto which is poured the collodionthat will form the membrane. This support firmly maintains the activemembrane and damps the transmission of some of the mechanical stressthat could affect it. The hydrodynamic conditions when pouring this typeof flat membrane can thus be adapted to minimize the role of theproduction speed. The stress is partly absorbed by the support in placeof the collodion, which allows preserving the physicochemical propertiesof collodion during the membrane production process. However, the activesurface of the resulting flat membrane comes in contact with numerouselements as it is advanced and rinsed. The quality of its active surfacedeteriorates, and many defects consequently affecting its filtrationperformance are formed on this surface.

Similarly, there are existing hollow fibers having an outer skin whichare reinforced by a braided support to strengthen their radial andlongitudinal mechanical strength. However, the active skin of thesefibers is on their unprotected outer surfaces and they are thereforeconsiderably damaged by their contact with the various advancement meansduring production. In addition, contact with the support affects theexternal-to-internal filtration of hollow fibers having an outer skin.

With the proposed method, the advantage due to using a support isapplied to hollow fiber membranes having an inner skin, without thedisadvantages mentioned above concerning flat membranes associated witha support and concerning hollow fibers having an outer skin that areassociated with a support. A membrane with hollow fibers having an innerskin forming a plurality of channels so supported and protectedpreserves the intrinsic membrane properties resulting from the nature ofcollodion while avoiding defects on the outer surface of said hollowfibers having an inner skin. External-to-internal filtration of hollowfibers having an inner skin is not impacted by contact with the support.This combination of support/hollow fibers having an inner skin alsogives the membranes improved performance compared to known membranes.Furthermore, this novel membrane production technique has never beendescribed nor implemented.

The method enables the production, at high spinning speed, of membraneswith hollow fibers having an inner skin providing a plurality ofchannels. Indeed, the nature of the rinsing bath is a vital factor forhollow fibers having an inner skin. To manage the porosity of the outersurface of the fiber with precision, water/solvent mixtures must be usedto control the phase separation that occurs on the outer surface of thefibers. However, the use of such mixtures generally reduces the fiberspinning speed considerably and requires the use of a rinsing bath ofvery large volume. With our novel form of membranes, the ratio of porevolume/effective filtration area is minimized because the thickness ofthe porous section which surrounds the filter channel is significantlyreduced. Precipitation of the membrane is then faster because it dependson the overall thickness of the membrane, and the time required forrinsing the membrane is reduced at the same time.

The implementation of the method can thus produce a membrane containingone or a plurality of channels formed by the hollow fibers having aninner skin and supported by at least one support film, this support filmpreferably being a nonwoven support. In one particular embodiment, thesame support may be directly welded to a drainage mesh which willtherefore be on the outer surface of the membranes so produced.

Multi-channel membranes manufactured according to embodiments of themethod can be produced in parallel using a production chain that isparticularly simple and inexpensive.

The channels of the hollow fibers so produced have a relatively broadrange of diameters. Channels having diameters preferably between 0.6 and1.5 mm can thus easily be produced, with the possibility of extendingthe range of diameters to 0.2 to 3 mm.

In one particular embodiment, a second support film is applied andunrolled at the outlet of the extrusion spinneret, thereby improving themechanical strength of the flat hollow-fiber membrane.

To protect the nascent hollow fiber at the outlet of the extrusionspinneret, the membrane is positioned between the first and secondsupport film. It is then immediately protected from the hazards of themanipulations and contacts it would be exposed to if these supports didnot exist. The characteristics of the support film may be selectedaccording to the filtration performance of the membranes to be produced.It is therefore unnecessary to use a thick and robust support if themembrane is intended for use at low pressures or if the base collodionallows producing a membrane with strong mechanical properties. Thesupports may be treated according to the collodion composition used; itis possible to dry the support or wet it with a suitable solventbeforehand in order to control collodion penetration. This allowscontrolling the connection between the polymer matrix of the membraneand the porous support used to strengthen it.

In comparison to hollow fibers produced with identical formulations, thelinear speed of production of membranes produced according toembodiments of the method can be increased by approximately 50% or more.This is due to the decreased thickness of the membrane to beprecipitated and formed. Therefore the production capacity can bedoubled or tripled for example. In addition, with an effective membranewidth of 40 mm, it is possible, for example, to simultaneously producetwenty to forty channels in parallel. Compared to conventionalproduction conditions, the production capacity is multiplied by a factorof between 2 and 8 depending on whether the number of filaments producedin parallel is 16 or 8 filaments. Advantageously, with the proposedmethod, the resulting membranes are protected and maintained, and havestrictly controlled separation performance.

Moreover, the method eliminates the influence of the composition of theexternal bath, which here only serves as the rinsing bath and willusually be minimized because the membranes will be rinsed by sprayingthem with water through injection nozzles.

The rest of the manufacturing operation can proceed as required by themanufacturing process: finishing the rinsing, posttreatment, and storingthe membranes after drying for later production of filtration modules.This manufacturing method makes it possible to produce membranes whileminimizing the waste of materials used to produce a module having agiven filtration area. If so required by the manufacturing process, themembranes leaving the first rinsing bath are fed to continuous washingelements then bundled or wound onto spools. The membranes are then giventheir final wash, then posttreatment and drying before grouping theminto modules for the bonding operation.

In one particular embodiment, the internal liquid may comprise degassedwater alone or mixed with another substance. The degassed water makes itpossible to obtain excellent results when implementing the method.

The addition of a solvent or other additive to the internalprecipitation liquid allows regulating the characteristics of thefiltering skin produced, according to many known publications that arein the public domain.

Preferably, during step b), the collodion and the internal liquid can bebrought to a temperature between 30 and 60° C. These temperatures enableoptimum implementation of the method.

In one particular embodiment, the support film is, for example, anonwoven polypropylene-based film. This choice is explained by thestrength and the low cost of such a support. However, the use of apolyester-based support may be advisable depending on the requirementsof the application.

Advantageously, the membrane with hollow fibers having an inner skinpresenting a plurality of channels exiting the extrusion spinneret canbe set in motion by the effect of a tractive force applied to thesupport film with which it is associated. Such a pulling force may, forexample, be obtained by an assembly of two rollers placed at each end ofthe membrane. These rollers may be located on either side of thespinneret and close to the membrane exit. In this case, the two supportsare pulled, by their lateral ends, at a constant speed into a bathcontaining a liquid which begins rinsing the membrane; however, theycould also be sprayed by the rinsing liquid in order to minimize theimplementation cost of this bath.

Advantageously, the filtration membrane may be cut after step e) so thatsaid filtration membrane has closed or open inner surfaces. When itleaves the rinsing bath, the film may be cut automatically to producemembranes of a given length. The cutting tool can thus provide closed oropen inner surfaces depending on manufacturing requirements.

In one particular embodiment, the method for manufacturing a filtrationmembrane may comprise an additional step of crushing the filtrationmembrane to facilitate bending it. It is thus possible to leave areas ofsmaller width with no filtration channels, to allow bending the film socreated. The location of these areas is chosen according to thegeometric characteristics of the module to be produced. In the case of aplate/cartridge module, the presence of these areas is not necessary.

A filtration membrane is also proposed, comprising at least one hollowfiber having an inner skin composed of an organic polymer and associatedwith a support film. Such a filtration membrane preferably comprises aplurality of hollow fibers having an inner skin and allows obtainingparticularly advantageous filtration performance.

Preferably, the support film is of a porous nature, which allowsimproving the filtration of treated solutions without compromisingmembrane transfer and strength.

A final aspect concerns the use of a filtration membrane as definedabove, and preferably produced according to the method of the invention,for the manufacture of a filtration module. Such a module hasparticularly advantageous properties for the filtration of water or forseparation, concentration, or purification of any other fluid.

BRIEF DESCRIPTION OF DRAWINGS

Other features and advantages of the invention will become apparent uponreading the following description. This is purely illustrative andshould be read with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic example of a spin assembly for implementing aconventional extrusion method,

FIG. 2 is a front sectional detail view of an extrusion spinneret forimplementing a conventional extrusion method,

FIG. 3 is a front sectional view of an extrusion spinneret forimplementing the proposed method,

FIG. 4 is a side sectional view of the extrusion spinneret shown in FIG.3, for implementing the method,

FIG. 5 is a side sectional view of the extrusion spinneret forimplementing the method, representing the application of the supportfilms,

FIG. 6 is a cross-sectional view of a filtration membrane with a singlerow of channels obtained after implementing the method according to theinvention,

FIG. 7 is a cross-sectional view of a filtration membrane with two rowsof channels obtained after implementing the method,

FIG. 8 is a cross-sectional view of a filtration membrane with threerows of channels obtained after implementing the method,

FIG. 9 is a cross-sectional view of a filtration membrane with two rowsof channels of large diameter, obtained after implementing the method,

FIG. 10 is a cross-sectional view of a filtration membrane with a singlerow of channels and a crushed buffer area, obtained after implementingthe method.

DETAILED DESCRIPTION OF EMBODIMENTS

The description of embodiments of the method is given below withreference to some examples.

Example 1—Membranes Based on Polyethersulfone (PES)

A spin assembly as shown in FIG. 1 can be used for the production ofconventional filtration membranes by extrusion. Such an assemblycomprises a tank of collodion 1 and a tank of internal liquid 2, bothconnected to an extrusion spinneret 3 that allows forming a nascentfiber. This nascent fiber falls into a precipitation bath 4 and, bymeans of take-up rollers 4 b is, is guided toward a rinsing bay 5 and abundling machine 6 which is used to roll it. For experimental spinning,the bundling machine 6 is not used and the nascent fiber falls directlyinto a basin of water extending to the exit from the large roller of therinsing bay 5, which advances it forward.

To manufacture conventional membranes based on polyethersulphone (PES),collodion can be prepared from a polymer mixture containing 16% Veradel3100P polyethersulfone in the presence of 6% polyvinylpyrrolidone K30 inN-methylpyrrolidone, while stirring and at a temperature maintained at80° C.

The collodion is then filtered through metal mesh (5 micron filtrationrating) and transferred to a storage tank where it is vacuum-degassedand then stored at a temperature of about 50° C.

The internal precipitation fluid in this example is water that isdegassed by ultrasound and then stored prior to use in a tank maintainedat, for example, 50° C.

The hollow fiber is produced with the collodion and internal fluid at atemperature of 50° C. A specific extrusion spinneret is used to producea fiber having an inner diameter of 0.85 mm and an outer diameter of1.45 mm. The spinning speed used is 16 m/min.

FIG. 2 illustrates the flow of the collodion 1 and of the internalliquid 2 within the extrusion spinneret 3 in more detail. The internalliquid 2 is injected into a centering needle 7 in which the channel ispositioned at the core of the extrusion spinneret at its axis. Thechannel of the extrusion spinneret is fed collodion 1 upstream of thecentering needle 7 so that the internal liquid 2 is fully covered by thecollodion 1 when exiting the extrusion spinneret.

The precipitation bath 4 which begins the fiber rinse is filled withwater maintained at 40° C. The fiber exiting the bath is advanced by alarge motorized roller located at the rinsing bay 5 which releases itinto a basin filled with water. The water temperature is also maintainedat 40° C. in this basin, where the rinsing of the fiber and thus theremoval of residual solvent is completed.

After 24 hours of soaking in water, a long piece of fiber is cut toproduce a module containing 12 fibers. The total length of thismicromodule is equal to 30 cm, which represents an effective filtrationlength of 26 cm and a filtration area of 83 cm².

An initial test measurement of the water permeability of the membranesshows that the fibers thus obtained are only slightly permeable towater. In order to regain permeability in these fibers, two solutionsare considered:

1) soaking the produced hollow fibers in water at 80° C. for 10 hours,2) soaking the produced hollow fibers in water at 30° C. containing 500ppm of NaClO (pH adjusted to 10) for 24 hours.

The permeability measurements in water brought to 20° C. (Lp20) in amicromodule of fibers treated in this manner are as follows:

Lp20 (L/h · m² · bar @ 20° C.) Fibers rinsed with water at 80° C. for 10hours 650 Fibers soaked in water at 30° C. with 500 ppm 720 of NaClO (pH10)

The rise in the water permeability index observed here is mainly due toelimination of the free polyvinylpyrrolidone trapped between the polymernetworks of polyethersulfone.

With the same collodion 1, it is possible to produce a filtrationmembrane according to the invention, formed of 33 channels covered withtwo nonwoven supports having the following characteristics:

-   -   nonwoven polypropylene-based support of technical quality,        specially processed for use as a membrane support.    -   support thickness of 60 microns.    -   a weight index of the support of 17 g/m².

An extrusion spinneret as depicted in FIG. 3 can thus be used in themethod of the invention. In this FIG. 3, the frontal section shows theupstream orifices 8 of the extrusion spinneret through which thecollodion 1 flows and then traverses an upper portion 9 of the extrusionspinneret. This upper portion 9 is connected to a lower portion 10 bymeans of screws 11. The internal liquid 2 is injected via centeringneedles 7 inside the extrusion spinneret, through the side port 12. Inthis example, 33 centering needles are distributed in a row along theextrusion spinneret, to create a filtration membrane having 33 channels.

FIG. 4 shows a side sectional view of the arrangement of one of thecentering needles 7 at the core of the extrusion spinneret, in thecenter of the lumen traversing said extrusion spinneret and throughwhich the collodion 1 flows.

The above precipitation conditions are used. The conditions of thecollodion and internal liquid flow rates are fixed so as to produce amembrane with channels 0.85 mm in diameter. Two nonwoven supports(effective width of 35.2 mm and total width of 46 mm) are applied to thenascent multi-bore membrane in order to obtain a total thickness of 1.38mm as shown in FIG. 6.

FIG. 5 illustrates the application of the support film onto the nascentfiber, resulting in formation of the filtration membrane according tothe method of the invention. The support films 13, in this case thenonwoven materials of this example, unwind and pass through variousrollers 14 positioned at the perimeter of the extrusion spinneret andpressing said nonwoven supports 13 onto the outer walls of the nascentfiber exiting the extrusion spinneret. The support film immediatelyadheres to the nascent fiber flowing from the extrusion spinneret, dueto capillary action and the wetting by the solvent used to prepare thecollodion.

The two nonwoven supports serve to protect the fibers forming thechannels of the membrane of the invention. The membrane so formed isproduced at a spinning speed of roughly 16.5 m/min. This membrane isthen advanced by a set of two rollers placed on both sides of thespinneret outlet. The membrane then slides directly into the U of twoflattened half-tubes prepared and placed on both sides of the spinneretand which are oriented in the direction in which the membrane exits.This imposes a specific path for a distance that can be varied accordingto the rate of precipitation of the membrane. In the present case, thepath is 3 m, which is sufficient for solidification of the nascentmembrane and continuing with the other manufacturing steps. The membraneof the invention is then conveyed to a cutting member, which cuts itinto lengths of 1.4 m that fall into a basin containing water where therinsing continues.

Samples of these membranes are subjected to the same tests as above andare used to fabricate micromodules containing a single membrane that hasan effective length of 26 cm (total length of 30 cm and filtration areaof 229 cm²).

Permeability measurements at 20° C. are as follows:

Lp20 (L/h · m² · bar @ 20° C.) Untreated NovaMem membrane +/−0 NovaMemmembrane rinsed with water at 80° C. 1500 for 10 hours Fibers soaked inwater at 30° C. with 500 ppm 1640 NaClO (pH 10)

One can see that the water permeability of the membranes of theinvention is higher than that of conventional hollow fibers. One willalso note that the conventional hollow fiber, although it hassatisfactory mechanical properties (breaking load: 7 N-elongation atbreak: 45%), requires more care during manipulation than the filtrationmembrane of the invention which is protected by the nonwoven backing. Inaddition, the membrane of the invention is considerably more robust yetflexible and is manipulated via the support that surrounds it andprotects its outer surfaces. Moreover, the covering of the contactsurface of the support film with collodion is controlled so as to form amembrane where its performance is linked to the composition of thecollodion and to the process conditions used during spinning, and is nolonger dependent on the advancement conditions used, furthercontributing to maintaining said performance.

For reference purposes, a 300 DN module of effective filtration lengthof 1.2 m and a fill factor of 55% provides a filtration area equal to:

-   -   70.5 m² with conventional hollow fibers as described above.    -   73 m² with filtration membranes according to the invention        produced as described above.

In both cases, a ring 5 mm thick is installed on the inner circumferenceof the housing in order to distance the fibers and membranes from theinner surface of the housing which requires bonding. In the case of themembrane of the invention, the bonding area per membrane is 1.37×38 mm.Therefore, only 693 membranes, each individually protected and eachcontaining 33 filter channels, are used to produce this module insteadof 22,000 hollow fibers. This allows concluding that a high level ofsecurity is achieved with this novel manufacturing method.

The method thus enables the production of qualitatively more reliablemembranes and also the creation of filtration modules that provide amore advantageous filtration area, the created filtration channels beingproduced in a smaller space.

Example 2—Membranes Based on Polyacrylonitrile (PAN)

To manufacture conventional membranes based on polyacrylonitrile (PAN),a polymer (collodion) mixture is prepared containing 18%polyacrylonitrile in the presence of 2% lithium chloride inN-methylpyrrolidone, while stirring at a temperature maintained at 70°C.

The collodion is then filtered through a wire mesh (5 micron filtrationrating) and transferred to a storage tank where it is vacuum-degassedand then stored at a temperature of 40° C.

The precipitation liquid is water, degassed by ultrasound and thenstored prior to use in a tank maintained at 40° C.

The hollow fiber is produced with the collodion and internal fluid at atemperature of 40° C. A specific extrusion spinneret is used to producea fiber having an inner diameter of 0.90 mm and an outer diameter of1.62 mm.

The spinning speed used is 18 m/min.

The external rinsing bath is filled with water maintained at 40° C. Thefiber exiting the bath is advanced by a motorized roller which releasesit into a basin filled with water. The water temperature is alsomaintained at 40° C. in this basin, where the rinsing of the fiber andthus the removal of residual solvent is completed.

After 24 hours of soaking in water, a long piece of fiber is cut toproduce a module containing 12 fibers. The total length of thismicromodule is equal to 30 cm, which represents an effective filtrationlength of 26 cm and a filtration area of 91 cm².

Measurement of water permeability in the micromodule yields a constantwater permeability index equal to 290 l/h·m²·bar at 20° C. The fibersobtained in this manner have a breaking load equal to 7.6 N and anelongation at break equal to 45%. Although these mechanical propertiesof elongation appear satisfactory, the crushing strength of these fibersappears low (feel flexible when touched, and crush quickly).

With the same collodion, a filtration membrane according to theinvention is produced that is formed of 33 channels covered with twononwoven supports having the following characteristics:

-   -   nonwoven polypropylene-based support of technical quality,        specially processed for use as a membrane support.    -   support thickness of 95 microns.    -   weight index of the support is 34 g/m².

The above precipitation conditions are used. The conditions of thecollodion and internal liquid flow rates are fixed so as to produce amembrane with channels 0.90 mm in diameter. Two nonwoven supports(effective width of 37 mm, total of 46 mm) are applied to the nascentmulti-bore membrane in order to obtain a total thickness of 1.49 mm. Asthe mechanical strength of the nascent polyacrylonitrile membraneprovides insufficient crushing strength, a thicker nonwoven support of95 microns is used although it is quite possible to obtain a membrane ofthe invention having satisfactory mechanical performance with a supportonly 60 microns thick.

The two nonwoven supports serve to protect the fibers forming thechannels of the filtration membrane of the invention. The membrane soformed is produced at a spinning speed of roughly 18 m/min. Thismembrane is then advanced as explained in Example 1, to be rinsed andcut into lengths of 1.4 m which fall into a basin containing water wherethe rinsing continues.

Samples of these membranes are then collected in order to fabricate afiltration module containing a single membrane that has an effectivelength of 26 cm (total length of 30 cm) and a filtration area of 243cm².

As above, the water permeability of the filtration module just producedaccording to the invention is measured, to find that the waterpermeability index of the membrane is equal to 850 l/h·m²·bar at 20° C.One can see here that the difference between the water permeability ofthe membrane of the invention and of a conventional hollow fibermembrane is greater than with the fibers produced in Example 1. InExample 1 the ratio Lp_(Invention)/Lp_(Fiber) was close to 2.3, while inthis example the same ratio is close to 2.9.

This is explained by the fact that the rate of precipitation of thecollodion used to produce PES membranes is faster than that of thecollodion used to produce PAN membranes. In technical terms, the PANnascent fiber falls into the rinsing bath when its outer surface has notcompletely finished gelling, so that the rinsing bath has moreinfluence. In the case of the PES fiber, the outer surface of thenascent fiber is in a more advanced state of gelification. Here, therinsing bath has less impact on the fiber performance.

To explain this phenomenon, we can say that the nascent fiber must beproduced at a sufficiently slow spinning speed for it to be immersed inthe rinsing bath only when its outer structure has solidified. Themanufacture of membranes according to the method of the invention allowsus to reach this state because it allows us to reduce the thickness ofthe porous support formed around the filter channels. In the case ofhollow fibers, the thickness of the fiber must be fairly high in orderto give it sufficient crushing strength. The nonwoven backing we useoffers two advantages. Firstly, it protects the membrane surfaces fromthe handling devices as mentioned above. In addition, it also appearsthat decreasing the thickness of the membrane obtained according to themethod of the invention has a positive impact on the filtrationperformance of the membrane.

We can show that a 300 DN module of effective filtration length of 1.2 mand a fill factor of 55% produces a filtration area equal to: 60 m² withconventional hollow fibers as described above.

68 m² with membranes according to the invention produced as describedabove.

In both cases, a ring 5 mm thick is installed on the inner circumferenceof the housing in order to distance the fibers and membranes from theinner surface of the housing which requires bonding. The bonding area ofthe membrane according to the invention is 1.49×40 mm. Therefore 610membranes containing 33 filter channels are used instead of close to17,600 conventional fibers.

On the other hand, in order to produce PAN hollow fibers, the diameterratio is increased to 1.8 instead of the 1.7 for PES fibers. This givesthe PAN fibers satisfactory crushing strength. With the method accordingto the invention, it is possible to maintain an equivalent ratio (1.62for the PES membrane and 1.65 for the PAN membrane). For the membrane ofthe invention, this ratio is the ratio of the total thickness of themembrane and the channel diameter.

Example 3—Filtration Modules

The table below gives the filtration areas that can be provided by afiltration module manufactured according to the invention, having aninner diameter of 300 mm and an effective filtration length equal to1,200 mm. Three configurations of membranes of the invention areconsidered:

1—A membrane 15 as illustrated in FIG. 6, having a single series ofthirty-three channels 16 (Series 1) placed between two layers of supportfilm 17 measuring 1.5 mm in total thickness, which is a bonding area of1.5×40 mm. This membrane is suitable for all potential cut-offs, frommicrofiltration to nanofiltration.

2—A membrane 15 as illustrated in FIG. 7, having two series of thirtychannels 16 (Series 2) placed between two layers of support film 17measuring 2.7 mm in total thickness, which is a bonding area equal to2.7×40 mm. This membrane is more suitable for high ultrafiltration,ultrafiltration, or nanofiltration applications.

3-. A membrane 15 as illustrated in FIG. 8, having three series ofthirty channels 16 (Series 3) placed between two layers of support film17 measuring 3.7 mm in total thickness, which is a bonding area of3.7×42 mm. This membrane is more suitable for ultrafiltration andnanofiltration applications.

The filtration areas were calculated for two potential fill factors ofthe module (55 and 60%), while reducing the inner radius of the housingby 5 mm in order to place a centering ring allowing better adhesion ofthe head plate.

TABLE Filtration areas provided according to fill factor and number ofchannels Number of Number of Fill factor membranes per Filtration area *channels (%) module (m²) Series 1 33 55 605 68 60 660 74 Series 2 60 55336 68 60 367 75 Series 3 90 55 234 71 60 255 78

We can immediately see that the filtration area which can be providedwith membranes of the invention far exceeds that achievable withconventional hollow fibers of the same inner diameter. The advantage ofthis geometry is not limited to this aspect. Indeed, the number ofmembranes used is clearly reduced, and they have fibers that areparticularly well-protected. In addition, the time taken to produce themembranes needed to equip the S3 module is less than 20 min, a veryshort time compared to standard values, despite a linear spinning speedequal to 20 m/min. This is also achieved while fully protecting theintegrity of the membrane, since the filtration surface of the membraneand its outer surface do not come into contact with any element thatcould affect integrity. Similarly, the tools for producing this novelgeneration of membranes are limited to the members for preparing andfeeding the collodion and internal precipitation liquid. Finally, themembrane can be advanced during manufacture by members mounted at itsends and which can securely adhere the two support layers. However, thisis not always necessary because the compression of the two supports bythe two drive rollers and the fiber covering which constitutes thesupport film may be sufficient to ensure complete adhesion of themembrane to the supports so applied. Finally, the path of the nascentmembrane within the two slides (flattened U-shaped tube) positioned oneon each side, forms a rinsing path that maintains membrane integritybetter than any known method.

Example 4—Filtration Membrane of the Invention Having Channels of LargeDiameter

In the case of hollow fibers of large diameter, the crushing andbursting strength of the fibers requires a substantial thickness. For afiber having an inner diameter of 2.7 mm, the outer diameter must be atleast equal to 5 mm. This creates many difficulties related to thefollowing:

-   -   First, the amount of collodion used is quite large: for one m²        of effective area produced, close to 2.3 liters of collodion        will be used, excluding waste.    -   The nascent fiber is manipulated very carefully, as the fiber        quickly bends and may flatten whenever it touches a handling        device. This forms a fiber which has an oval shape and unequal        thickness, which is very fragile when compressed or crushed.    -   The outer skin of such a fiber requires using a very low        spinning speed in order to be as independent as possible of the        rinsing bath composition.        Other difficulties relate to the mechanical strength of the        fibers during use. A minimal loss of fiber integrity can easily        result in fracture propagation from a starting point of        fracture, which affects retention of the products to be stopped        by the membrane. In contrast, it is possible to produce a        membrane according to the invention equipped with numerous        filtering channels having a diameter equal to 2.7 mm and to use        a thinner wall. This is possible due to the presence of the        support film acting as a protective layer advantageously lined        with a separating mesh (here acting as a reinforcement), which        gives the membrane the following advantages:    -   The support layer and its irrigation mesh become a layer of        mechanical reinforcement, which reduces the thickness of the        membrane wall without any risk of reducing the crushing strength        of the membrane. In the present case, a protective layer having        a total thickness equal to 0.250 mm is used, of which 0.050 mm        is provided by the mesh.    -   FIG. 9 shows a membrane 15 according to the invention equipped        with twenty-two channels 16 that are 2.7 mm in diameter. This        membrane is made with a support film 17 having a total thickness        of 7.7 mm and a width of 42 mm (bonding area 7.7×46 mm). To        produce one m² of this membrane, only 1.1 liters of collodion        are used, excluding waste: this is a 52% reduction in the amount        of collodion, replaced in part by the support which is formed of        less technical material but better reinforces the mechanical        strength of the membrane and its stability over time.

Note that the time required to produce 1 m² of membrane according to theinvention is at least halved. Although some membrane manufacturers areable to produce up to 16 hollow fibers in parallel, the method of theinvention can increase the linear production speed by 50 or even 100%.In addition, the method of the invention ensures that the membrane, andmore particularly the hollow fibers thereof, are not damaged by thehandling devices, which is not the case with other known manufacturingmethods. Finally, the membrane channels formed by the hollow fibers socreated are perfectly cylindrical, while individually produced hollowfibers of large diameter are often flattened or given an oval form bytheir contact with the advancement means. This last detail is veryimportant because such deformed fibers age very badly and fairly quicklyend up generating mechanical fractures that propagate, similarly to awelded tube split along its length.

Example 5—Filtration Membrane of the Invention with Buffer Zone

FIG. 10 illustrates a membrane according to the invention, created withan intermediate buffer zone. In this membrane 15, formed for examplewith one row of channels, fifty channels 16 of 0.9 mm are placed along atotal effective width of the support film 17 that is equal to 70.7 mm(which is a bonding area of 1.5×75 mm). As before, the thickness of thismembrane is equal to only 1.5 mm, which allows providing a largefiltration area per unit volume.

This configuration provides an important advantage:

1—For an effective filtration length equal to 1.2 m, the filtration areafor each membrane element is 0.17 m².

2—In a housing which has an inner diameter of 300 mm, it is easy toobtain a total filtration area of between 55 and 60 m² depending on thefill factor applied (respectively 55 and 60%).

3—Although some of the available filtration area is lost per module, theproduced membranes are fitted into the module more quickly (+50%), andproduction is managed with a more compact and more productive tool.

4—To produce a module providing 60 m² of filtration area, only 352properly protected membranes are manipulated rather than 19,200self-supporting, fragile hollow fibers that require more collodion.

Many opportunities therefore exist for producing diverse forms, eachproviding specific advantages. The main and key advantage of the methodof the invention lies in the fact that it ensures production offiltration membranes in the manner that best provides the appropriateperformance. This concept also allows producing membranes which haveunmatched hydraulic and mechanical properties.

1. A flat filtration membrane, comprising: a first protective supportfilm and a second protective support film, and a plurality of hollowfibers having an inner skin delimiting respectively a plurality ofparallel filtration channels, the hollow fibers being formed by anorganic polymer, wherein the first and second protective support filmsare applied on outer walls of the hollow fibers.
 2. The filtrationmembrane according to claim 1, wherein at least one of the first andsecond protective support film is a nonwoven support.
 3. The filtrationmembrane according to claim 1, wherein at least one of the first andsecond protective support film is of a porous nature.
 4. The filtrationmembrane according to claim 1, wherein at least one of the first andsecond protective support film are polypropylene-based.
 5. Thefiltration membrane according to claim 1, wherein the inner skinincludes a small width area without channels, so as to ease a folding ofthe filtration membrane.
 6. The filtration membrane according to claim1, wherein each filtration channel has a diameter comprised between 0.2and 3 mm.
 7. The filtration membrane according to claim 1, having aratio of the total thickness of the membrane and the channel diametercomprised between 1.6 and 1.8.
 8. A flat filtration membrane,comprising: a first protective support film and a second protectivesupport film, a plurality of hollow fibers having an inner skindelimiting respectively a plurality of parallel filtration channels, thehollow fibers being formed by an organic polymer, wherein the pluralityof hollow fibers are positioned between the first and second protectivesupport films.
 9. A filtration membrane that includes a plurality ofchannels and is made a method comprising: a) selecting and preparing acollodion, b) injecting the collodion into at least one channel of anextrusion spinneret that further comprises a core and an outlet, c)injecting an internal liquid into a plurality of hollow centeringneedles, wherein each centering needle comprises a channel positioned atthe core of the extrusion spinneret, d) applying a first protectivesupport film at the outlet of the extrusion spinneret, unrolling thefirst protective support film onto a first face of a flat filtrationmembrane emerging from the outlet of the extrusion spinneret, thefiltration membrane having a plurality of filtration channels, e)applying a second protective support film at the outlet of the extrusionspinneret, unrolling the second protective support film onto a secondface of the filtration membrane emerging from the outlet of theextrusion spinneret, f) immersing the filtration membrane with the firstand second support films in a rinsing solution, g) ending the rinsing ofthe filtration membrane.